It means that a *whistle carrier comes up to the edge of the drum and blows his whistle for the drum to sing 4 more leads/verses. For some reason the video you posted does not show the whistle carrier. You can only hear his whistle being blown at the end of the video.

*There is controversy in modern powwows on who has the right to blow whistles at a powwow. In olden times it was a honored position in grass dance and warrior societies. Not everyone had this right. Today, times change and the lines of tradition have been blurred.